The present invention relates to reflective liquid crystal display devices.
As liquid crystal display devices are thin and light, they are used in a wide variety of applications including displays for portable information terminals. Liquid crystal display devices are passive devices that do not emit light but display information by changing intensity of light transmission. As they can be driven with a root-mean-square voltage of several volts, when used as a reflective type by installing a reflector underneath a liquid crystal display device so as to make use of the ambient light, back-light power is made unnecessary thus providing a display device with extremely low power dissipation.
Existing reflective color liquid crystal display devices comprise a liquid crystal cell with a color filter and a pair of polarization films disposed in a manner such that they sandwich the liquid crystal cell. The color filter is provided on one of the substrates of the liquid crystal cell, and a transparent electrode is formed on top of the color filter. Color display is effected by controlling light transmissivity of each of the color filter elements by changing the alignment of liquid crystal molecules through application of a voltage to the liquid crystal cell.
Light transmissivity of a polarization film is at most about 45% and the light transmissivity of polarized light in parallel to the axis of absorption of the polarization film is almost 0%, while that of polarized light perpendicular to the absorption axis is about 90%. Consequently, in a reflective liquid crystal display device using two polarization films, the maximum reflectivity neglecting absorption by the color filter and other components is calculated to be:
(0.9)4xc3x9750%=32.8%
as the light is reflected out after passing the polarization films four times. Thus, even the reflectivity of a black-and-white display panel which does not use a color filter is at most about 33%. Incorporation of a color filter reduces the reflectivity to about ⅓ of this value.
Accordingly, in an effort to obtain a brighter display, a few proposals have been made on a configuration in which the liquid crystal cell is sandwiched between a polarization film and a reflecting plate as in, for example, Japanese Laid-Open Patent Applications No. Hei 7-146469 and Hei 7-84252. In this configuration, as the light passes the polarization film only twice, the maximum reflectivity neglecting absorption by the color filter and other components is calculated to be:
(0.9)2xc3x9750%=40.5%,
which is about a 23% improvement at the maximum over a configuration using two polarization films.
However, in a liquid crystal cell which uses one polarization film, color distortion tends to occur when trying to make color display using a color filter as well as to securing brightness by increasing reflectivity, thereby presenting a difficulty in making achromatic display of black and white, especially difficulty in displaying low-reflectivity, achromatic black.
There have also been proposed a reflective color liquid crystal display apparatus (Japanese Laid-Open Patent Application No. Hei 6-308481) in which multicolor display is effected by using birefringence of twisted nematic liquid crystal layer and a polarization film without using a color filter, or a color liquid crystal display apparatus (Japanese Laid-Open Patent Application No. Hei 6-175125 and Japanese Laid-Open Patent Application No. Hei 6-301006) which makes use of the birefringence of the liquid crystal layer and phase difference film. As these apparatuses do not use a color filter, a reflectivity value enough to obtain practically acceptable brightness can be secured even when two polarization films are used. However, as the color display uses coloring based on birefringence, it has been difficult, by principle, to realize multi-shade, multi-color displays such as displays of 16 shades of 4096 colors or full-color displays with 64 shades. They also suffered narrow ranges of color purity and color reproduction.
In view of these circumstances, it is an object of the present invention to provide a reflective liquid crystal display device which gives bright display of white and a high contrast, and which is capable of displaying achromatic black as well as multi-color, multi-shade displaying.
The reflective liquid crystal display device according to the present invention includes a liquid crystal cell made by sealing nematic liquid crystals between a pair of substrates, a polarization film disposed on one of the substrates of the liquid crystal cell, a phase plate disposed between the polarization film and the liquid crystal cell, and an optical reflection means disposed on the other substrate, where the twist angle of the nematic crystals between the pair of substrates is in the range 0xc2x0 to 90xc2x0, the product xcex94nLCxc2x7dLC of the index of birefringence xcex94nLC of the nematic liquid crystals and the thickness dLC of the liquid crystal layer is in the range 0.20 to 0.30 micrometer, the difference in the indices of birefringence xcex94R as defined, using the product xcex94nLCxc2x7dLC and retardation RF of the phase plate, by RFxe2x88x92xcex94nLCxc2x7dLC is in the range xe2x88x920.20 micrometer to xe2x88x920.05 micrometer, defining the direction in which nematic liquid crystals are twisted from one of the substrates toward the other substrate when viewed from one of the substrates is positive in angle, the direction of the major axis of liquid crystal molecules closest to one of the substrates as "PHgr"LC, the direction of the slow axis of the phase plate by "PHgr"P, the direction of the absorption axis or the transmission axis of the polarization film by "PHgr"P, the present invention is featured by either of the following relations, that is, "PHgr"F-"PHgr"LC is in the range xe2x88x9240xc2x0 to xe2x88x9225xc2x0 and "PHgr"P-"PHgr"F is in the range +50xc2x0 to +80xc2x0, or "PHgr"F-"PHgr"LC is in the range +65xc2x0 to +105xc2x0 and "PHgr"P-"PHgr"F is in the range xe2x88x9260xc2x0 to xe2x88x9290xc2x0.
Here, as taking of "PHgr"P in the direction of the absorption axis of the polarization film is optically equal to taking it in the direction of the transmission axis, either direction is acceptable.
Also, in the reflective liquid crystal display device in accordance with the present invention, it is preferable that the twist angle of nematic liquid crystals is in the range 30xc2x0 to 65xc2x0 and RF is in the range 0.10 micrometer to 0.30 micrometer. These preferred examples will provide further improved characteristics.
With this configuration, a normally white reflective liquid crystal display device which is capable of displaying bright image with achromatic black and white, and multiple colors in multiple shades.